1. Field of the Invention
The present invention relates to an optical element comprising a birefringence layer and a process for producing the same, a substrate for liquid crystal alignment having a birefringence layer, and a liquid crystal display device comprising the substrate for liquid crystal alignment. The present invention also relates to a birefringent material.
2. Background Art
Liquid crystal display devices have advantages including that a reduction in thickness and a reduction in weight can be easily realized, the power consumption is low, and the occurrence of flicker can easily be prevented. By virtue of these advantages, liquid crystal display devices have drawn attention as flat panel displays, and the market of liquid crystal display devices for use as display devices of personal computers or television receivers have been rapidly expanded. Further, an increase in size of the liquid crystal display device is also being forwarded.
For these liquid crystal display devices, various display modes have been developed. Since liquid crystals have birefringent properties, any display mode of liquid crystal display devices basically has visual angle dependency. In large-size liquid crystal display devices, the practical visual angle is larger than that in small-size liquid crystal display devices. Therefore, an increase in size of liquid crystal display devices leads to an increasing demand for an improvement in visual angle dependency. To meet this demand, the development of liquid crystal display devices have led to the development of various techniques for improving the visual angle characteristics.
Conventional liquid crystal display devices having a visual angle increased by controlling liquid crystal alignment include, for example, liquid crystal display devices of a multi-domain system in which the liquid crystal within pixel is divided into a plurality of regions different from each other in alignment direction at the time of at least display of intermediate tones, and liquid crystal display devices of an IPS (in-plane switching) system in which liquid crystal alignment is controlled by forming transverse electric field (electric field parallel to substrate surface) within the liquid crystal cell.
Further, in order that the visual angle is increased by optically compensating light incident on a liquid crystal cell or light emitted from the liquid crystal cell, various optical elements having birefringent properties have been developed. Liquid crystal cells for optical compensation or optical compensation films comprising an optically monoaxially or biaxially stretched resin film have hitherto been used as the optical element. In recent years, an optical element having a birefringence layer formed of a liquid crystal material or a process for producing the same have also been developed.
For example, Japanese Patent Laid-Open No. 142531/1993 discloses a visual angle compensation film comprising nematic liquid crystal polymer having a positive inherent refractive index value in which molecular chains have been aligned in a direction normal to the film face.
Japanese Patent Laid-Open No. 174724/2002 discloses that a vertically aligning film using a long-chain alkyl-type dendrimer derivative is formed and a polymerizable liquid crystal compound is coated onto the vertically aligning film to form a homeotropically aligned liquid crystal layer and, thus, to prepare an assembly which is then used as an optical film.
Japanese Patent Laid-Open No. 174725/2002 discloses a production process of an optical film. This production process comprises coating, on a substrate not provided with any vertically aligning film, a side chain-type liquid crystal polymer comprising monomer units containing a liquid crystalline fragment side chain and monomer units containing a non-liquid-crystalline fragment side chain, then homeotropically aligning this liquid crystal polymer in a liquid crystal state, and then fixing the molecular arrangement while holding the alignment to prepare an optical film.
Japanese Patent Laid-Open No. 121852/2003 discloses a production process of an optical film. This production process comprises providing a binder layer and an anchor coat layer in that order on a substrate, then coating a specific side chain-type liquid crystal polymer on the anchor coat layer, homeotropically aligning the liquid crystal, and then fixing the molecular arrangement while holding the alignment to prepare an optical film.
However, in order to provide the visual angle compensation film described in Japanese Patent Laid-Open No. 142531/1993, a method should be used in which two substrates each having a vertically aligning film are used to prepare an empty cell, the empty cell is filled with a nematic liquid crystal monomer, the liquid crystal monomer is homeotropically aligned and then photopolymerized, and the nematic liquid crystal polymer (visual angle compensation film) should be taken out from within the cell. Therefore, the visual angle compensation film described in this literature is disadvantageous in that the production process is complicated and the production cost is high.
In order to provide a homeotropic alignment-type liquid crystal layer by the process disclosed in Japanese Patent Laid-Open No. 174724/2002, the use of the vertically aligning film is indispensable and a special and unobtainable material, i.e., a long-chain alkyl-type dendrimer derivative, should be used for vertically aligning film formation. For this reason, the production process disclosed in this literature is disadvantageous in that the production cost is sometimes high.
The homeotropic alignment-type liquid crystal film produced by the process described in Japanese Patent Laid-Open No. 174725/2002 comprises a side chain-type liquid crystal polymer. Therefore, the birefringence characteristics are susceptible to heat, and, thus, the temperature range in which desired birefringence characteristics can be maintained is relatively narrow. This makes it difficult to use the homeotropic alignment-type liquid crystal film, for example, in an on-vehicle liquid crystal display device where relatively high heat resistance is required. Further, the homeotropic alignment-type liquid crystal film produced by the process described in this literature is disadvantageous in that liquid crystal display devices using the homeotropic alignment-type liquid crystal film can be used only in limited applications.
Further, in the side chain-type liquid crystal polymer, even when the molecules are fixed in a homeotropically aligned state, the fluidity is increased with increasing the temperature. This results in lowered adhesion to the substrate layer or a significant lowering in birefringence characteristics due to residual stress. Therefore, when the homeotropic alignment-type liquid crystal film is used in liquid crystal display devices, in order to avoid exposure of the once formed liquid crystal film to a high-temperature environment, a liquid crystal film should be provided on the outer side of the liquid crystal cell after the production of the liquid crystal cell. Thus, the homeotropic alignment-type liquid crystal film produced by the process described in Japanese Patent Laid-Open No. 174725/2002 is disadvantageous in that the degree of freedom of selection of the member which can form this liquid crystal film is low.
The homeotropic alignment-type liquid crystal film produced by the process described in Japanese Patent Laid-Open No. 121852/2003 comprises a side chain-type liquid crystal polymer and thus suffers from the same problem as described above. Further, in order to provide the homeotropic alignment-type liquid crystal film by the process described in this literature, a binder layer and an anchor layer should be provided in that order on the substrate, and, thus, the production cost is disadvantageously increased.